1. Field of the Invention
The present invention relates in general to a fluid treatment process and to the tablets used to effect such treatment. In particular, the present invention relates to a chlorination or dechlorination process in the potable water and wastewater treatment field, where the chlorination/dechlorination process is effected by placing stacked tablets of a chlorination or dechlorination agent in the flow path of the fluid to be treated. The tablets are formed in a shape that causes the dissolution rate of the tablets in the fluid to remain substantially constant during the water treatment period.
2. Background and Description of the Related Art
Chlorine tablets have been extensively applied in commercial or smaller private potable water and wastewater treatments plants, as have dechlorination tablets. The tablets are normally stacked in a tube or other feed device which extends into the water to be treated. The bottom tablets in the feed device are initially immersed in the water, while the upper stacked tablets remain out of the water. The water flows past the bottom tablets, gradually dissolving these tablets as the water is treated. When the bottom tablets have dissolved to the point at which they can no longer support the upper tablets, the remains of the bottom tablets collapse and the remaining tablets in the feed device advance toward the bottom of the feed device, submersing some of the upper tablets in the water. These submersed tablets are now the bottom tablets, and the cycle is repeated.
Water treatment processes utilizing stacked tablets in a tube shaped feeder device are well known. U.S. Pat. No. 4,816,177 to Nelson et al. discloses a tubular device for holding stacked tablets used for the treatment of water. U.S. Pat. No. 3,595,786 to Horvath et al., U.S. Pat. No. 4,732,689 to Harvey et al., and U.S. Pat. No. 4,759,907 to Kawolics et al. all disclose similar tablet feed devices.
Most stacked tablets used for fluid treatment are cylindrically shaped with flat opposing surfaces. Because of the flat opposing surfaces, the entire top surface area of any one tablet in the stack is always completely in contact with the entire bottom surface area of the tablet resting on top of it. This tablet shape in a stack of other similar tablets results in two phenomena adverse to uniform dissolution and distribution of chemicals in a fluid: pillaring and wicking.
When a tablet feeder which is loaded with stacked tablets is placed in the fluid to be treated, the fluid contact surface area of the bottom tablets is maximized. After a few hours of operation, the periphery material of the bottom tablets is dissolved into the treated fluid and the diameter of the bottom tablets gradually becomes smaller. The direction of tablet erosion is from the periphery of the tablet toward the center. Gradually, a central pillar which still supports the upper tablets is formed at the bottom of the tablet stack. While the upper tablets are held by the pillar, the fluid contact surface area of the smallest pillar may be only 30 to 45 percent of its initial surface area. The dissolution rate of the tablet is directly proportionate to its fluid contact surface area. The dissolution rate is therefore much lower during the pillaring period. It is therefore not reliable to set or adjust the goal dissolution rate based on initial tablet surface area. The actual dissolution rate will vary over the course of the fluid treatment, and the lower dissolution rate during the pillaring period may not meet design or discharge requirements for the treated water.
The contact surface between two flat-surfaced tablets is a surface which also offers a large area for moisture diffusion from the water line to the upper tablets. Gradually, a wicking problem is caused by moisture saturation. That is, water will saturate the portions of the bottom tablets which have not yet dissolved. Because of the large surface contact area between adjacent tablets, some of this saturated moisture will quickly diffuse into the upper tablets in a process known as a wicking effect. The effect of this diffusion is that the upper tablets will not be completely dry when they are eventually submersed in the water, and therefore will dissolve and break apart too rapidly. For a constant dissolution rate to be achieved, the wicking effect must be minimized by reducing the diffusion speed.
The tablets used with the feeder disclosed in U.S. Pat. No. 4,816,177 to Nelson et al. are of a round cylindrical shape with flat opposing surfaces. U.S. Pat. No. 3,856,932 to May also discloses round, cylindrical tablets for fluid treatment having flat opposing surfaces. May recognized that fluid treatment tablets must be modified in order to achieve the goal of a constant dissolution rate. Rather than change the shape of the tablet, however, May coated a portion of the surface of the tablet with a fluid impervious material in order to slow the initial dissolution rate. This solution slows dissolution but does nothing to prevent the pillaring or wicking effect in feeder tablet delivery.